For many years there has existed a desire for polyolefin materials which can be bonded to other materials, e.g., metals, ceramics and wood, or to themselves with epoxy resin adhesives. Bonding of such materials with epoxy resin adhesives is often desirable, particularly when the bonded material is to be subjected to conditions of high temperature and high humidity.
Untreated polyolefin substrates, e.g., polyethylene, polypropylene or polyallomer which is a copolymer of polyethylene and polypropylene, are generally unreceptive to epoxy adhesives and much effort has been expended in the search for a technique which will easily and reliably increase adhesion without significantly changing the properties of the substrate. Surface modification techniques for enhancing adhesive bonding which have been examined are treatment with helium gas plasma, oxygen gas plasma and chromic acid. These treatments and other surface modification procedures have a common shortcoming in poor durability. Light rubbing of the surface causes a decrease in the effect, the altered surface being easily abraded.
Irradiation of polyolefin substrates, such as with an electron beam, to improve the adhesion of various coatings is also known and has been disclosed in U.S Pat. No. 4,041,192 (Heger et al.), U.S. Pat. No. 4,148,839 (Fydelor), U.S. Pat. No. 3,252,880 (Magat et al.) and U.S. Pat. No. 4,179,401 (Garnett et al.).
Polyolefin substrates have been provided with a treated surface for improved adhesion of pressure-sensitive adhesives. Such pressure-sensitive adhesives include rubber-type adhesives such as those based on natural rubber and synthetic rubbers, e.g., styrene-butadiene, chloroprene, neoprene, and isobutylene rubbers and synthetic resin-type adhesives such as acrylic, polyvinyl chloride, polyvinyl acetate, and polyvinyl butyral adhesives. U.S. Pat. No. 3,628,987 (Nakata et al.) discloses a pressure-sensitive adhesive film wherein the film surface to which the adhesive is adhered has graft-polymerized thereto a vinyl monomer or diene monomer, the adhesive having a solubility parameter near that of polymers of the vinyl or diene monomer. U.S. Pat. No. 4,563,388 (Bonk et al.) discloses a polyolefin substrate having graft-polymerized thereto at least one monomer selected from the group consisting of acrylic acid, methacrylic acid and esters thereof; acrylamide; methacrylamide; sterically non-hindered tertiary alkyl acrylamides and methacrylamides; secondary alkyl acrylamides and methacrylamides having three or less carbon atoms in the alkyl group; and N-vinyl pyrrolidone, and firmly adherently bonded to the graft-polymerized monomer an acrylic type normally tacky and pressure-sensitive adhesive.
A series of articles, "Surface Modification of Polyethylene by Radiation-Induced Grafting for Adhesive Bonding. I. Relationship Between Adhesive Bond Strength and Surface Composition," (S. Yamakawa, J. Appl. Polym Sci, 20, 3057-3072 (1976); "II. Relationship between Adhesive Bond Strength and Surface Structure," (S. Yamakawa et al , Macromolecules, 9, 754-758, (1976); "III. Oxidative Degradation and Stabilization of Grafted Layer," (S. Yamakawa et al., J. Appln. Polym. Sci., 22, 2459-2470 (1978); "IV. Improvement in Wet Peel Strength," (S. Yamakawa et al., J. Appl. Polym. Sci., 25, 25-39 (1980), and "V. Comparison with Other Surface Treatments," (S. Yamakawa et al., J. Appl. Polym. Sci., 25, 40-49 (1980), disclose grafting of methyl acrylate (followed by saponification), vinyl acetate, acrylic acid, acrylamide, and methylolacrylamide to polyethylene by vapor-phase mutual grafting or liquid-phase preirradiation at thicknesses of grafted monomer of more than 10 micrometers to improve adhesion of epoxy adhesives.